1. Field of the Invention
The present invention relates to an image pickup apparatus capable of executing flash-photographing, a camera system, and a control method for the image pickup apparatus.
2. Description of the Related Art
Conventionally, various techniques have been proposed for flash-emission control of image pickup apparatuses (such as a camera and a digital still camera) at flash-photographing. Among these, there are many proposals to perform preliminary light emission of a flash prior to an exposure operation and photometrically measure reflection light from a subject at the preliminary light emission for each of divided regions of an imaging screen (a subject image observed through a finder or a display), thereby deciding an amount of main flash. This is because an amount of light emission for various photographing scenes can properly be decided by deciding the amount of main flash in accordance with predetermined algorithm based on the result of photometric measurement on the divided regions.
With regard to the flash-emission control, an image pickup method has been proposed to stably obtain an appropriate amount of exposure (for example, Japanese Laid-open Patent Publication No. 2005-275265). In this method, a ratio R(i) between light metering values P(i) and H(i) immediately before and during preliminary light emission is calculated for each of light metering areas. Then, a maximum ratio among the calculated ratios R(i) is extracted as a reference value baseR, and a value of R(i) for each light metering area is compared with the reference value baseR to thereby decide a weighting coefficient W(i) for each light metering area. Next, amounts of reflection light received by respective ones of the light metering areas at the preliminary light emission are weight-averaged using the weighting coefficients W(i) to thereby obtain a weighted average of the amounts of reflection light at the preliminary light emission, and an amount of main flash is calculated based on the obtained weighted average.
An image pickup method has also been proposed for detecting a human face region to attain a satisfactory exposure, even if subject brightness is insufficient (see, for example, Japanese Laid-open Patent Publication No. 2005-184508). In this method, it is detected whether or not there is a human face region in an image picked up at preliminary flash emission. Next, a light control region is decided based on the detected face region, and an amount of main flash is calculated based on a light metering value in the light control region at the preliminary light emission.
Also, an image pickup method for flash-emission control has been proposed (see, for example, Japanese Laid-open Patent Publication No. 2003-075717). In this method, a predetermined subject image (specifically, a human face) is extracted from an image pickup signal obtained upon image pickup of the subject, and a distance to subject is calculated based on the size of the extracted subject image. The calculated result is used for flash-emission control, etc.
With the technique disclosed in Japanese Laid-open Patent Publication No. 2005-275265, it is possible to attain a stable exposure at many photographing scenes and reduce a change in exposure in a case that the composition is slightly changed, with the photographing scene remaining the same. In this technique, a light metering area having a maximum ratio among ratios R(i) for light metering areas that empirically satisfy a predetermined condition is regarded as a main subject area, and R(i) of such area is extracted as a reference value baseR and used for decision of weighting coefficients W(i) and calculation of an amount of main emission. On the other hand, a large weighting coefficient W(i) tends to be assigned to a light metering area corresponding to a subject located at close distance or having a high reflectance. Therefore, if a light metering area corresponding to a true main subject does not satisfy the predetermined condition, the weighting coefficients W(i) and the amount of main emission become inappropriate, and the exposure intended by the photographer cannot be provided.
With the technique disclosed in Japanese Laid-open Patent Publication No. 2005-275265, an amount of main flash is calculated based on an amount of reflection light from the subject at preliminary light emission. Thus, if the subject has a high or low reflectance, weighting coefficients W(i) dependent on the reflectance of the subject and the amount of main emission become inappropriate. Thus, the resultant exposure can be deviated from a preferred exposure in some cases.
With the technique disclosed in Japanese Laid-open Patent Publication No. 2005-184508, a satisfactory exposure can be obtained with high possibility in a case that a main subject is a human and a human face is reliably detected. However, if the resolution of a sensor used for photometric measurement of reflection light at preliminary light emission is inadequate for the size of a face region in the imaging screen, a problem is posed that when an attempt is made to select the face region as the light control region for determination of the amount of main flash, influences of a background around the human subject or other regions cannot be removed, sometimes resulting in inappropriate exposure.
With the technique disclosed in Japanese Laid-open Patent Publication No. 2003-075717, it is highly possible to attain a satisfactory exposure not dependent on the reflectance of the subject by calculating a photographing distance to subject based on the size of the subject's face and deciding an amount of emission according to the calculated distance. In the case of flash-photographing, it is generally possible to calculate the required amount of flash emission based on distance-to-subject information and a diaphragm value for photographing, if accurate distance-to-subject information is obtainable. Since the thus calculated amount of flash emission provides information not dependent on the reflectance of the subject, it is preferable to obtain the distance-to-subject information.
With most of the state-of-art technologies, however, even a face image, etc. printed on a poster or other printed material is detected as a human's face. Since the size of the face image on a printed material does not necessarily coincide with the size of an actual face, it is highly possible that a photographing distance calculated from the subject size has a large error, resulting in unsatisfactory exposure at the time of photographing.
There is an idea to obtain the distance-to-subject information by use of an auto-focus mechanism of a camera or the like. However, the auto-focus mechanism has a detection error in distance measurement depending on various conditions. In addition, there is a production error in a distance encoder that provides information on the position of a distance ring of a taking lens. Thus, a problem is posed that it is difficult to determine whether or not the obtained distance-to-subject information includes a small error or a large error relative to the actual distance to subject.